This work was supported by the National Science Foundation under NSF Award Number DMR-0632947 and the Nanoscale Science and Engineering Initiative under NSF Award Number EEC-0647560. This work made use of the NUANCE Center facilities, which are supported by NSF-MRSEC, NSF-NSEC and the Keck Foundation. We thank G. C. Schatz for many useful discussions. T.W.O. is a DuPont Young Professor, an Alfred P. Sloan Research Fellow, a Cottrell Scholar of Research Corporation, and a David and Lucile Packard Fellow.